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Working principle of continuously variable transmission

Updated:18-08-2021
Summary:

There is no specific gear position, and the operation i […]

There is no specific gear position, and the operation is similar to an automatic gearbox, but the change in speed ratio is different from the automatic gearbox skipping process, but is continuous, so the power transmission is continuous and smooth.

There are various structures such as V-shaped rubber belt type, metal belt type, multi-disc type, steel ball type, and roller turntable type. Most of them use metal belts and variable-radius rollers to transmit power. Through the change of the radius of the active roller and the passive roller, the gear ratio can be changed. In theory, the efficiency of this type of transmission is very high, but it must be based on the ability to load the transmitted power. Because the friction force between the steel belt and the roller is used to transmit power, the working conditions of the steel belt and the roller are very harsh. In order to effectively transmit power, no slipping is allowed between the steel belt and the roller, and the original heat energy is already a lot, if it slips again, it may cause internal parts to burn or severely wear. In order to increase the static friction, the most direct way is to increase the pressure between the steel belt and the roller. However, as friction increases, the loss of power transmission will increase, and fuel consumption will increase virtually. And the strength of the steel strip is also an important point. Therefore, CVT gearboxes have the advantages of comfort, high efficiency and energy saving. The disadvantage is that the current general CVT gearbox cannot withstand large torque. Otherwise, it is necessary to compensate with higher fuel consumption.

The difference between a continuously variable transmission (CVT: Continuously Variable Transmission) and a stepped type is that its gear ratio is not a discontinuous point, but a series of continuous values, for example, it can vary from 3.455 to 0.85. The structure of CVT is simpler and smaller than traditional transmission. It has neither the numerous gear pairs of manual transmission nor the complicated planetary gear set of automatic transmission. It mainly relies on the main and driven wheels and metal belts to achieve stepless change in speed ratio.
The principle is that several sets of gears of different sizes, like ordinary gearboxes, are divided and combined under control to form different speed ratios. Like the pedals of a bicycle, the wheels are driven to rotate at different speeds through the large and small wheels and chains. Because different forces produce different thrusts for each group of gears, the speed output by the gearbox also changes accordingly, so as to achieve slow rotation regardless of grades.

CVT uses a transmission belt and a variable groove width ratchet for power transmission, that is, when the groove width of the ratchet changes, the contact radius of the driving wheel and the driven wheel on the transmission belt is changed accordingly. The transmission belt generally uses rubber belts, metal belts and metal chains. CVT is truly stepless. Its advantages are light weight, small size, and fewer parts. Compared with AT, it has higher operating efficiency and lower fuel consumption. However, the shortcomings of CVT are also obvious, that is, the transmission belt is easily damaged and cannot withstand large loads. It can only be used in low-power and low-torque cars with a displacement of about 1 liter, so the automatic transmission occupancy rate is about 4% or less . In recent years, after vigorous research by major automobile companies, the situation has improved. CVT will be the development direction of automatic transmissions.

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